The present invention relates to an electrically weldable socket, sleeve or bushing for joining pipe members, and in particular to such a member made from thermoplastic material having a resistance heating wire embedded therein close to its inner wall in the form of spaced turns or helix, the member is partly softened together with the piece to which it is connected by the heat produced by the resistance heating wire and is welded with the latter.
Pipe systems are now frequently produced by using thermoplastic pipe members. Pipe members are understood to mean pipe lengths, shaped articles, control members, e.g. valves and other means. The problem arises of assembling the pipe members and interconnecting them in a drip- or leak-proof, pressure-tight and tension-resistant manner at their connection.
Cylindrical sockets with sleeve-like socket bodies made from thermoplastic material, which are welded together with the pipe member connecting pieces are known. The socket member can form part of a pipe member or can be a separate part. For the purpose of partially melting and welding the adjacent surfaces of the parts to be connected, an electrical resistance heating wire is embedded in the vicinity of the inner wall of the socket body in the form of a helix. Through the supply of electric power to this wire, a metered quantity of heat is produced which is sufficient for joining the socket member and the connecting piece. For the purpose of metering this quantity of heat welding units are used. Different constructions of such units are known, so that e.g. on setting a constant voltage or current a specific welding time is obtained.
The socket weldable by means of such welding units would appear to be a relatively simple member. However, it must be ensured that on embedding the heating wire, the individual turns are separated from one another by a sufficiently large amount of plastic, so that interturn short-circuits are reliably prevented. To ensure a good connection of the parts to be welded together, the socket body is given a shrinkage reserve, which is eliminated during the welding process and as a result the socket body engages tightly with the parts to be joined. Whereas during welding the socket body decreases in size, the resistance heating wire, which is heated during the supply of electric power, expands counter to the shrinkage movement of the socket body.
These two oppositely directed movements can bring about a bending of individual turns of the heating wire and can consequently cause a short-circuit, leading to the thermoplastic material of the socket body being damaged or catching fire. The resulting linear expansion of the resistance heating wire also results in the movement of the individual turns away from the welding area and their spacing can become so large that the complete melting of the inner wall within the welding zone may not be possible. Such phenomena are more particularly observed with electrically weldable sockets with a large socket diameter, e.g. over 200 mm.
Different measures have been proposed for obviating these problems. Thus, e.g. the total length of the heating wire can be reduced by using a plurality of individual short helices. This reduces the overall length and expansion of the heating wire, as well as the risk of interturn short-circuits. However, the circuitry expenditure is increased.
It is also known to use a varnish or tape-insulated heating wire, whose insulation is intended to prevent a short-circuit of the interturn type. However, the insulation of commercially available resistance heating wires is such that it is not adequate for effectively preventing interturn short-circuits. There is also a danger that the insulation carbonizing at the higher temperatures of the resistance heating wire, will thereby give off solvent vapours which can cause cavities to form in the welding area.
In order to eliminate the difficulties which more particularly occur when welding large pipe diameters, it has been proposed (in German publication DOS 2,854,618) to leave a sufficient gap between the socket body and the connecting pieces of the pipe members to be connected to receive a welding sleeve and a ring around which is wound the resistance heating wire. Through producing heat in the heating ring, the ring having a shrinkage reserve is deformed in such a way that the welding sleeve presses against the surfaces of the socket and the connecting piece to be joined, followed by softening and welding. However, this connection is time consuming and costly and requires a relatively large amount of space.
In U.S. patent application Ser. No. 129,993, filed Mar. 13, 1980, now abandoned, in which one of the present inventors is also a co-inventor, it was proposed to use a single mono-filament resistance wire having a low coefficient of thermal expansion permitting the thermoplastic material to melt and weld prior to the expansion thereof.
It is the object of the present invention to so develop an electrically weldable socket of the aforementioned type so that a completely satisfactory trouble-free welded joint is reliably contained, even in the case of large pipe diameters, whilst only involving a limited manufacturing expenditure.
These objects and others together with numerous advantages are obtained by the present invention as set forth in the following disclosure thereof.